1. Field of the Invention
The present invention relates to a sealing material for sealing an envelope of an electron tube using P2O5—SnO type low melting glass powder, and a method of preparing the sealing material for sealing an envelope of an electron tube, as well as, to an electron tube having an envelope sealed with the sealing material.
2. Description of the Related Art
FIG. 8 is a diagrammatic view showing a general structure of a vacuum fluorescent display, as an example of the electron tube, having an envelope sealed by a sealing material. The vacuum fluorescent display 1 shown in FIG. 8 generally includes the envelope which is maintained in high vacuum. The envelope is formed of a glass substrate 2 on which an anode electrode and a grid electrode are formed, a spacer glass plates 5, and a front panel 6 bonded together with an amorphous low melting glass 7 as a sealing material for sealing the vacuum fluorescent display. The envelope further includes an anchor for supporting a filament and various metal parts, such as, a filament lead 3 for taking the filament out of the envelope and a lead 4 for taking the anode electrode and grid electrode out of the envelope to be bonded between the upper surface of the glass substrate 2 and the spacer glass plates 5 with the amorphous low melting glass 7.
The amorphous low melting glass 7 used in the vacuum fluorescent display is a paste-like sealing material prepared by kneading a mixture of PbO—B2O3—SiO2 type low melting powder and thermal expansion controlling ceramics PbTiO3 with a vehicle prepared by dissolving a binder, such as, 1˜5% ethyl cellulose in a solvent, such as, terpineol.
The amorphous low melting glass 7 as a sealing material is formed in a predetermined pattern on the upper surface of the glass substrate 2 in advance by means of a screen printing method, and then subjected to a baking step in which temperature is raised to about 400˜500° C., maintained at that temperature, and then lowered to ordinary temperature to be vitrified to form the seal pattern 8.
The seal pattern 8 is heated again to about 400˜500° C. at assembly steps of the envelope to bond and hermetically seal the glass substrate 2 to the spacer glass plates 5 and then the temperature is lowered to ordinary temperature. As a result, the seal pattern 8 is made to be amorphous glass, and the hermetic sealing can be maintained.
In recent years, in order to minimize adverse effects on environment, it has been proposed to use numerous lead-free low melting glass powders including various phosphoric acid type low melting glass powders instead of the PbO—B2O3—SiO2 type low melting glass powder containing Pb which is hazardous to environment. For example, these glass powders are disclosed in Unexamined Japanese Patent Publications No. 5-132339 and No. 8-239239.
A sealing material comprising P2O5—SnO type low melting glass powders as a phosphoric acid-type low melting glass powders is disclosed in, for example, Unexamined Japanese Patent Publication No. 11-283537. A paste-like sealing material prepared by kneading the P2O5—SnO type low melting glass powders with a vehicle is disclosed in, for example Unexamined Japanese Patent Publication No. 2000-72749. Further, a paste-like sealing material prepared by kneading the P2O5—ZnO—SnO type low melting glass powders with a vehicle, such as, nitrocellulose, used for preparing a good hermetically sealed envelope by providing a temperature profile maintaining 200˜250° C. for ten (10) minutes is disclosed in, for example, Unexamined Japanese Patent Publication No. 2002-20319.
In accordance with the teachings of prior art, inventors prepared a paste-like sealing material by kneading 80 wt % of phosphoric acid type lead-free glass (powder glass of P2O5—SnO type low melting glass having the softening point of about 300˜400° C.) with 20 wt % of vehicle prepared by dissolving 3˜5 wt % of binder, such as, ethyl cellulose in a solvent, such as, alcohol. The paste-like sealing material thus obtained is formed in the seal pattern 8 similarly to prior art and then maintained at temperatures of 400˜500° C. for a given time to form vitrified paste-like sealing material. The temperature is then raised again to, and maintained at, 400˜500° C. to melt the vitrified sealing material again, thereby hermetically sealing the glass substrate 2 and the spacer glass plates 5. However, it is noted that the mechanical strength of the sealing material at the sealing area is not sufficient.
In order to confirm and pursue the cause of the insufficient mechanical strength of the phosphoric acid-type lead-free glass (powder glass of P2O5—SnO type low melting glass having the softening point of about 300˜400° C.,) the inventors prepared the sealing material comprising PbO—B2O3—SiO type low melting glass powder having sufficient mechanical strength which is subjected to bake at a temperature of 400˜500° C. to form amorphous low melting glass. FIG. 1 shows an ×200 magnification micrograph of the surface of the amorphous low melting glass thus obtained.
As shown in the micrograph of FIG. 1, the amorphous low melting glass prepared by baking the sealing material comprising P2O5—B2O3—SnO type low melting glass powder at the temperature of 400˜500° C. includes non-vitrified surface of the amorphous low melting glass in part shown in the region (A) because of deposition of devitrified substances. However, a larger part of the surface of the amorphous low melting glass is vitrified as shown in the region (B).
On the other hand, the inventors prepared a sealing material comprising P2O5—SnO type low melting glass powder which is subjected to bake at a temperature of 400˜500° C. to form amorphous low melting glass. FIG. 2 shows an ×200 magnification micrograph of the surface of the amorphous low melting glass thus obtained.
As shown in the micrograph of FIG. 2, the amorphous low melting glass prepared by baking the sealing material comprising P2O5—SnO type low melting glass powder at the temperature of 400˜500° C. includes non-vitrified surface of the amorphous low melting glass on the whole region shown in the region (A) because of the crystallization of devitrified substances.
FIG. 5 shows relative content of substances present on the surface of the amorphous low melting glass prepared by baking the sealing material comprising P2O5—SnO type low melting glass powder at the temperature of 400˜500° C. shown in the micrograph of FIG. 2.
As shown in the graph of FIG. 5, each of SnO component which contributes to low melting point and P2O5 component constituting network structure on the surface of the seal pattern 8 using the conventional sealing material of P2O5—SnO type low melting glass powder decreases by about 50% or more as compared with those on the surface of the amorphous glass out of which devitrified substances are not crystallized.
Judging from the foregoing analysis, it is conceived that the weak mechanical strength of the sealing area of the conventional sealing material of P2O5—SnO type low melting glass powder is due to the crystallization of devitrified substance on the surface of the sealing material. Also, the weak mechanical strength of the sealing area of the amorphous glass prepared by baking the conventional sealing material of P2O5—SnO type low melting glass powder and thermal expansion-controlling ceramics at the temperature of 400˜500° C. is due to insufficient vitrification of the P2O5—SnO type low melting glass powder after baking at the temperature of 400˜500° C.
The reason why the P2O5 type low melting glass powder is not sufficiently vitrified is that SnO which contributes to low melting point as a network modifying agent penetrates partly the network structure of P2O5 and is crystallized in a state of glass having a stable structure intact at the time of baking the sealing material of P2O5 type low melting glass as a network molecule of glass at the temperature of 400˜500° C. As a result, devitrification occurs.